1. Field of the Invention
The present invention relates to a fuel injection control device applied to engines such as diesel engines and direct injection type gasoline engines.
2. Description of the Prior Art
A fuel injection control device for engines such as diesel engines has been known, in which an open-close valve provided in a fuel discharge passage for releasing fuel in a balance chamber is opened and closed by a solenoid actuator to control a pressure in the balance chamber and thereby control the lift of a needle valve that receives the fuel pressure in the balance chamber, optimumly controlling the amount of fuel to be injected and the injection timing according to the operating conditions of the engine, such as engine revolution and load.
The above fuel injection device has nozzle holes at the front end of the body for injecting fuel into the combustion chamber of the engine. A needle valve reciprocating in a hollow portion of the body opens and closes the nozzle holes with one end thereof. The fuel pressure in the balance chamber acts on the other end of the needle valve exposed in the balance chamber which forms a pressure receiving surface, to control the amount of lift of the needle valve (see Japanese Patent Laid-Open Nos. 965/1991 and 171266/1992 for example). The fuel pressure is supplied through supply passages into the balance chamber, whose pressure is released through the discharge passage. The open-close valve to open and close the discharge passage is driven by the solenoid actuator.
The applicant of this invention has proposed a fuel injection device with a control valve (Japanese Patent Laid-Open No. 77924/1998), in which the open-close valve installed in the discharge passage used to release the fuel in the balance chamber comprises a valve stem portion extending through the discharge passage into the balance chamber and a valve head portion provided at the front end of the valve stem portion and having a valve face that contacts a valve seat formed in the inlet side opening of the discharge passage to close the valve.
As to the control of fuel injection there is an increasing demand for increased fuel injection pressure to meet the requirements of emissions regulations, particularly the call for reduced amount of smoke.
During the idling where the amount of exhaust gases is relatively small, it is advantageous to lower the injection pressure for reduced vibrations and noise. An increased fuel injection pressure can disperse the injected fuel so that it can fully utilize not only the air present in the combustion chamber but the air in the cylinder bore as well, thus reducing the amount of smoke produced by incomplete combustion while at the same time meeting the conditions for high load operation. The high fuel injection pressure, however, increases the fuel injection rate causing sudden combustion, which in turn results in increased engine noise.
When the fuel injection pressure is reduced on the other hand, the low load operation can easily be dealt with. But during the high load operation that requires large fuel flows, the fuel injection period in one combustion cycle becomes longer, rendering the sprayed fuel not easily atomizable, deteriorating both the engine output and the exhaust gas characteristics.
Therefore, in a common rail pressure map that determines the common rail pressure, or fuel pressure in the common rail that stores fuel delivered from a fuel pump, it is common practice to set the fuel pressure high during the high load, high revolution operation and low during the low load, low revolution operation.
In a fuel injection device in which the valve head of the open-close valve in the form of a poppet valve is located on the chamber side, the open-close valve, when it is to be opened, needs to be pushed in toward the chamber side with a force stronger than the force produced by the fuel pressure in the chamber or the common rail-induced force. This drive force is required, because of the structure, to increase as the common rail pressure increases. Thus, the solenoid of the solenoid actuator is designed to produce a force enough to push in the open-close valve even when the common rail pressure reaches its maximum.
Designing the solenoid actuator in this way, however, results in driving the open-close valve with a large force provided for high common rail pressure even during the low load operation, such as idling, where the common rail pressure is set low. This produces injector noise, which consists mainly of impact noise between the control rod, which functions as the armature of the solenoid, and the stopper that restricts the displacement of the armature.
During the low load operation the pressure in the balance chamber is set low and the resistance against opening the open-close valve by the solenoid actuator is small. On the other hand, even when the attractive force of the solenoid is constant, the magnitude of the force is set large.
Hence, the initial armature displacement speed is high and the impact force of the armature striking the stopper is large. During the low load operation such as idling, in particular, because the combustion noise itself is small and there is no traveling noise that would be produced when running through the air and traveling on road, the impact noise between the armature and stopper can be very annoying.